Get length of array without using length

Question

In a completely contrived problem on exercism.io, I'm tasked to come up with a way to get the length/size of an array without using any enumerable methods.

I originally simply had:

  arr = [1,3,4]
  ctr = 0
  while arr[ctr]
    ctr += 1
  end
  ctr

The problem is that I can have arr = Array.new(5), which is [nil, nil, nil, nil, nil].

I've found two ways:

ctr = 0
loop do
  element = arr.fetch(ctr) { 'outofbounds!' }
  break if element == 'outofbounds!'
  ctr += 1
end
ctr

I'd like to do it without using Array#fetch because index out of bounds is just likely looking at the known length (which again I'm trying to implement).

Another solution:

  ctr = 0
  copy = arr.dup
  while copy != []
    ctr += 1
    copy.pop
  end
  ctr

This feels slightly right but == on Array first checks length then checks == on each element. I'm trying to implement length so stuck again.

@sawa thanks - I adjusted all the examples to use the same variables now. — Anthony
– Anthony, Commented Nov 7, 2017 at 13:25

tadman · Accepted Answer · 2017-11-07 02:50:56Z

2

Here's a solution that's deliberately really out there because this question is an exercise in not doing things using the core API:

def array_length(a)
  b = a.dup
  c = [ ]
  f = 0

  while (b != [ ])
    c.push('')
    b.pop
    f = 1
  end

  e = c.join("\x01").to_s.unpack('C*')

  while (e[0])
    f += e.pop
  end

  f
end

answered Nov 7, 2017 at 2:50

tadman

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Comments

Silvio Mayolo · Accepted Answer · 2017-11-07 03:38:44Z

1

I think your first solution is close. The problem, of course, is that nil is a valid element of the list. So the easiest solution is to add a sentinel value at the end which is never going to be an element of the list.

I've used this idiom from time to time; you just make a brand new instance of the Object class and use it as a value. No other object will ever compare equal to it, and it can't possibly be an element of the list since it was just now made up.

arr = [1,3,4]
end_of_list = Object.new
copy = arr.dup
copy.push end_of_list
ctr = 0
while end_of_list != copy[ctr]
  ctr += 1
end
ctr

edited Nov 7, 2017 at 3:38

answered Nov 7, 2017 at 2:45

Silvio Mayolo

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1 Comment

Anthony Over a year ago

Great suggestion

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